The present invention relates to a semiconductor memory device and, more particularly, to a technique effective when applied to a drawing processing RAM (i.e., Random Access Memory), for example.
A known drawing or picture processing RAM for displaying characters and images on the screen of a display device such as a CRT (i.e., Cathode Ray Tube) is exemplified by a serial access memory which is described in pp. 219 to 239, "Nikkei Electronics" of Nikkei McGRAW-Hill Corporation dated February 11, 1985. This RAM includes an address signal generating counter circuit which is operated by a control signal and a timing signal fed from an external terminal to generate a selection signal for selecting the lines of a memory array. This memory array has its data lines connected in parallel with a data register through a switch circuit so that data is serially transmitted between the data register and the external terminal. Since the data transmission to and from the external terminal is serially conducted, the picture element data synchronized with the raster scanning timing of the CRT can be easily extracted. However, the image processing random access memory thus far described is apparently a RAM but can only act as substantially a shift register having a bit number equal to the storage capacity of the RAM. Accordingly each memory cell is accessed only once each time all the bits are addressed. This raises a problem that the image processing operation followed by a preparation or change of an image is delayed in case a RAM of this kind is used.
For image processing, it is more convenient to use a RAM which can conduct a random accessing operation. We have conceived to construct an image processing RAM (i.e., a so-called "video RAM"). Such a RAM can be accessed as a unit of plural bits such as .times.4 bits (see for example the "Hitachi IC Memory Data Book" of Hitachi Ltd., issued in September, 1983). The aforementioned 4-bit signals are used for processing a color drawing or image and have red, blue, green and luminance signals assigned thereto. Even in case the RAM of that construction is used, however, the preparation or change of an image requires memory accessing operations and microprocessor operations over a plurality of cycles, which are effected by reading out picture element data once, by taking logical operations between the picture element data read out and new picture element data or display condition data, and by writing again the obtained picture element data to be changed in the initial memory address.